Communication in Distributed Systems

Tom Kelliher, CS43

Feb. 22, 1996

Communication Protocols

A set of rules followed by two processes (systems, people) which are communicating. The protocol governs how the communication is carried out.

Examples:

• A classroom
• The dinner table
• A diplomatic meeting

Circuit:

The pathway of communication between two computers. It may be a single wire, or a set of wires connected via switches (routers, gateways, bridges, etc.). The circuit may be dedicated or virtual.

Connection-Oriented protocols (TCP)

• Circuit set-up overhead
• Telephone service

Connectionless protocols (IP, UDP)

• US mail service
• Packet, message (store and forward) switching

The OSI Model

(Open Systems Interconnection Reference Model)

Seven layered abstract model of a protocol stack:

Advantages: modularity

Disadvantages: call overhead; size increases due to added headers, trailers

Physical Layer

• Switching levels, transmission rate
• Duplex
• Type of connector, cable

Ethernet:

• 10BaseT (UTP, 10Base2 (thinnet), 10Base5 (thicknet), 10BaseF (fiber)
• 10, 100Mbps

ATM:

• Fiber, SONET, Cat 5 UTP
• 45, 100, 155, 622Mbps; 2.5Gbps

Data Link Layer

• Partition the bit (packet) stream into frames
• Append checksums for error detection
• ``Guarantees'' that a frame makes ``one hop'', not that it's seen at the higher layers

Network Layer

• Routing multi-hop messages
• Connection-Oriented: X.25
• Connectionless: IP

Transport Layer

• End-To-End (possibly multiple-hops) reliability
• Partition the message stream into packets
• Message reassembly if built on a connectionless network layer protocol
• Connection-Oriented: TCP
• Connectionless: UDP

Session Layer

• Additional reliability features
• Often, not implemented (because it's thought of as the user process)

Presentation Layer

``Library''-type functionality

• Compression
• Encryption
• Character code conversion

Application Layer

User- System-Level utilities:

• ftp
• telnet, rlogin
• SMTP
• NFS
• DNS

Introduction to TCP/IP

Some of the protocols:

• Link layer --- NIC, device driver
• Network layer --- Routing, ``hop-by-hop'' messages
• Transport layer --- ``end-to-end'' messages

Message transmission example:

``Gluing'' Networks Together

• Repeaters/Hubs --- physical layer
• Bridges/Switches --- data link layer (ethernet)
• Routers --- network layer (protocol specific)
• Gateways --- protocol conversion

IP Addresses

Form:

• 32-bit, dotted decimal
• different from ethernet address (ARP, RARP)
• Class A: 0, 7 bit netid, 24 bit hostid

  0.0.0.0 to 127.255.255.255

• Class B: 10, 14 bit netid, 16 bit hostid

  128.0.0.0 to 191.255.255.255

• Class C: 110, 21 bit netid, 8 bit hostid

  192.0.0.0 to 223.255.255.255

• Domain Name System

Encapsulation

1. Data
2. Application layer
3. TCP/UDP layer --- TCP frame
4. IP layer --- IP datagram
5. Ethernet layer --- ethernet frame: 46--1500 bytes (MTU)

Demultiplexing

Process Communication

How do processes on separate machines communicate?

• Client/Server
• ``Well-known'' addresses
• Multiple telnet clients